It is known to use coupled waveguides to deliver optical radiation from one device or structure to another and to use characteristics of the coupled waveguides to improve that delivery.
For example, semiconductor optical devices historically have had very small waveguide modes and optical fibers have tended to have much larger waveguide modes, for instance of the order of tenfold. The mismatch in mode size leads to very low coupling efficiencies between waveguide and fiber.
It is known to use a secondary waveguide in order to expand the mode spot size of a semiconductor optical device. Examples are disclosed in U.S. Pat. Nos. 5,985,685 and 6,282,345. However, these have suffered from problems such as a relatively low fabrication yield, reproducibility and/or performance, which can lead to expensive production costs.